Photoelectrochemical water splitting by defects in nanostructured multinary transition metal oxides

Point defects play a crucial role in the performance of functional materials. The most important extrinsic elements for point defects in titanium-based photocatalysts are oxygen, nitrogen, and hydrogen due to their large chemical affinity and substantial solubility with titanium. Therefore, understanding the nature of such defects will help designing high-performance photocatalysts for various applications. Herein, we make use of alloyed multipodal Ti-Nb-Zr-O nanotubes (MPNTs) annealed under different atmospheres: Air, O-2, and H-2 for enhanced photoelectrochemical water-splitting. Structural analysis using XRD, Raman spectroscopy, and XPS confirmed the formation of a single mixed oxide Ti-NbZr-O in a strained-anatase crystal structure in both Air and Oxygen atmospheres. However, XPS fitting showed the presence of ZrTiO4 and Ti3+ upon annealing in Hydrogen atmosphere. Valence band XPS analysis confirms the presence of valence band tail states causing band-gap reduction in the hydrogen-annealed samples, with an absorption tail reaching NIR/Vis region. Mott-Schottky analysis showed 4 orders of magnitude increase in the carrier density for the samples annealed in hydrogen atmosphere compared to those annealed in Air or O-2, owing to the presence of Ti3+ defects/oxygen vacancies, titanium substitution by niobium, and the valence band tail states. These synergistic effects resulted in almost 25-fold enhancement in the photocurrent compared to the performance of the samples annealed in Oxygen or Air. It is thus concluded that annealing in a reducing atmosphere produces disordered and defective structure. Accordingly, the optical and electronic properties of complex metal oxides exhibiting poor performance can be manipulated to produce promising candidates for enhanced photoelectrochemical water splitting.